Allow an MJIT worker to show a backtrace on SEGV

An MJIT worker thread doesn't have ec, and it's required for SDR() and
rb_backtrace_print_as_bugreport(). Therefore it must be checked.

vm_dump.c

@@ -941,8 +941,9 @@ rb_vm_bugreport(const void *ctx)
     enum {other_runtime_info = 0};
 #endif
     const rb_vm_t *const vm = GET_VM();
+    const rb_execution_context_t *ec = GET_EC();
 
-    if (vm) {
+    if (vm && ec) {
 	SDR();
 	rb_backtrace_print_as_bugreport();
 	fputs("\n", stderr);